Several techniques exist to optically measure the distance of a target relative to a reference point. One of these techniques is laser triangulation. When sufficiently high numerical apertures can be used, laser triangulation techniques offer resolution at the micrometer scale. However, in the case where it is not possible to use large numerical apertures, laser triangulation techniques are less precise.
Time-of-flight techniques can also be used to determine the distance to a target. However, these techniques are based on using pulsed light sources and are generally used to measure long distances (in the kilometer range) with typical resolution at the meter range. Resolution of time-of-flight techniques can be improved when shorter light pulses and faster detection schemes are used, but a costly and complex measurement system is then required.
The Fourier transform analog to the time-of-flight technique is called Phase modulation telemetry. It proceeds by measuring the relative phase of the light modulated at frequency f, and coming back from a reflection on the target object. The phase difference can be related to the distance of the object. This technique has allowed demonstration of resolution in the range of 1 mm, but with an larger absolute distance value of the order of 5 mm due to the severe requirements on the phase value measurement at high modulation frequency.
Another way to measure a distance to a target is to use an interferometer with coherent light. Typical interferometers offer very good resolution (at the sub-wavelength range). However, the distance range that they measure is limited due to the phase ambiguity problem. The phase ambiguity can be removed by the use of dual wavelength sources as suggested by many authors. However, the requirement on the wavelength stability and the phase locking of both sources make the systems costly and unreliable.
One way to measure distances in the range of 0 to 20 mm is to use an optical sensor based on low-coherence interferometry. Low-coherence interferometers use a light source having a broadband intensity spectrum (in comparison to highly coherent sources such as lasers which have very narrow intensity spectrum) to determine the position of a target. In one of such existing systems, the spectrum intensity is coupled to a scanning Michelson interferometer. The scanning Michelson interferometer has a reference arm and a measuring arm, the measuring arm being the arm used for measuring the distance to the target. In such existing systems, the length of the reference arm needs to be scanned by a mechanical means during the measuring process.
There is a need for an optical sensor for distance measuring of a target that could be used in narrow conduits, that could measure distances in the range of a few micrometers to a few millimeters with good resolution, and that does not require moving mechanical parts.